lm9831 National Semiconductor Corporation, lm9831 Datasheet - Page 32

• Manufacturer Part Number: lm9831
• Description: 42-bit Color, 1200dpi Usb Image Scanner
• Manufacturer: National Semiconductor Corporation

8.0 Scanning
8.1 Start Scanning - Initiating an Image Scan
An image scan is initiated by writing a Scan command to Register
07. The LM9831 will move the sensor forward the number of full-
steps specified in registers 4A/4B and begin scanning. Scanning
ends when the host writes a new command to the command reg-
ister (Idle, Paper Feed to Start or Paper Feed to End) or when
PAPER SENSE 1 or PAPER SENSE 2 changes state (if pro-
grammed to do so).
The line buffer is reset when the Scanning bit is SET, not when it
is cleared. The host can continue to read stored data out of the
line buffer after a scan has stopped.
Pixel data is read from configuration register address 00. Regis-
ters at other addresses can be read during a scan (to read the
LM9831’s status registers, abort the scan, etc.).
If for some reason you want to pause the scan for some length of
time and resume later, do NOT stop the scan (return to Idle). Sim-
ply stop reading pixel data. When the buffer fills up, the LM9831
will automatically stop scanning and turn off power to the stepper
motor (when the delay goes beyond the time specified in the Hold
Current Timeout register).
The last 2 bytes of every line is a status word indicating how
much data is in the image buffer at the time the status word was
written. This information is in the 8 LSBs of the status word, and
has the same format as Register 01.
8.2 Reconstructing 8 bit Image Data Received By the PC
When reconstructing an image from the stream of data received
from the LM9831, it is useful to know the format of the data. The
LM9831 does not perform deinterleaving on the pixel data, it
comes out exactly as the sensor sends it. Deinterleaving must be
performed on the host PC.
For a single output CCD/CIS that outputs one line of data with
colors alternating at the line rate, the output format is:
R
G
B
For a triple output CCD/CIS that outputs 3 lines of data (each x
pixels apart in the vertical direction) with colors alternating at the
pixel rate, the output would be:
R
1
1
1
1
suspend mode if USB cable is
not attached to scanner.
, B
, R
, G
, G
Optional - forces LM9831 into
LM9831 D+
(pin 84)
LM9831 D-
(pin 83)
2
2
1
Figure 44: Recommended USB Component Values
2
, B
, R
, B
, G
3
3
1
3
, B
, R
, R
, G
4
4
2
4
,..., B
,..., R
, G
,..., G
10pF 10pF
2
, B
n-2
n-2
n-2
2
, B
,..., R
, R
, G
n-1
n-1
n-1
, B
n-1
, R
, G
n
, G
LM9831 V
n
n
(line m + 2)
22
22
(line m)
n-1
(line m + 1)
, B
(pin82)
n-1
REGULATOR
, R
1M
1.5k
n
, G
n
, B
Connector
Connector
n
D+ USB
D- USB
32
with the Red data representing line m+x, the Green data repre-
senting line m, and the Blue data representing line m-x. “x” is the
separation between lines, which depends on the physical dis-
tance between the R, G, and B sensors and the rate at which the
sensor is moving over the image.
The length of a line of image data sent to the PC depends on sev-
eral factors:
• The range of pixels to be scanned (Data Pixels): Data Pixels =
• The horizontal resolution set in the configuration register
• The number of bits per pixel (1, 2, 4, or 8, called B), and
• The color mode: pixel rate (C=3) or line rate (C=1).
The scanner software on the host must strip the 2 byte status
word from the end of each line before reconstructing the image.
8.2.1 Reconstructing 14 bit Image Data Received By the PC
In the 14 bit Data Mode the Gamma Correction and Pixel Packing
stages are bypassed. Each pixel comes out as 2 bytes instead of
1, doubling the amount of memory needed to store one line. The
data format is shown in Figure 45. This mode is otherwise identi-
cal to the 8 bit mode. The number of bytes per line in 14 bit mode
is given in this equation:
The 14 bit mode is used to acquire 14 bit data for accurate gain
and offset calibration.
8.3 High Speed Forward
When register 07 is set to a 1, the LM9831 moves the motor for-
ward at maximum speed (determined by the fast feed stepsize,
registers 48 and 49) until a 0 is written to register 07 or either one
of the PAPER SENSE inputs becomes True (if that sensor has
been properly programmed to interrupt scanner movement).
PAPER SENSE 2 can be used to cause a delayed stop. If the
FullSteps to Scan after PAPER SENSE 2 trips register is
greater than 0, motor movement will continue for the pro-
grammed number of full steps. This can be used to eject paper in
sheetfed scanners.
The LM9831 also features a Programmed High Speed Forward
command. This is identical to the High Speed Forward function,
except that it will automatically stop moving once the motor has
moved the number of lines specified in registers 4A and 4B.
8.4 High Speed Reverse
When register 07 is set to a 2, the LM9831 moves the motor
backwards at maximum speed (determined by the fast feed step-
size, registers 48 and 49) until a 0 is written to register 07 or
(Data Pixels End - Data Pixels Start),
(HDPI_Divider)
b13 b12 b11 b10
b5
7
Bytes/Line
b4
6
Bytes/Line
b3
5
Figure 45: 14 bit Data Format
=
b2
4
2 INT
=
b1
b9
2 INT
3
INT
----------------------------------------------------------------------- -
b8
b0
2
----------------------------------- -
HDPI_Divider
----------------------------------- -
HDPI_Divider
Data Pixels
Data Pixels
b7
1
0
16
b6
0
0
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Second Byte
First Byte
C B
C
Type